The present invention relates to a gas turbine power plant utilizing steam injection and a topping steam turbine. More specifically, the present invention relates to such a gas turbine power plant in which steam is generated in a heat recovery steam generator at higher and lower pressures.
The low capital cost, short lead times and flexibility of gas turbine-based power plants make them particularly attractive to electrical utilities as a means for generating electrical power. Unfortunately, the inefficiency of a gas turbine standing alone, referred to as a simple cycle system, is relatively low compared to conventional fired boiler steam turbine systems.
The major source of this inefficiency is inherent in the Brayton cycle on which the gas turbine operates. The ideal Brayton cycle operates in three phases--first, work is performed on the fluid (air in the case of a gas turbine) by isentropic compression in a compressor; second, heat is added to the fluid isobarically in a combustor; and, third, the hot compressed fluid is isentropically expanded back down to its initial pressure in the turbine. During the expansion phase much of the energy imparted to the fluid as a result of the compression and heating is recovered in the form of useful work. However, a significant portion of the energy remains in a relatively high-temperature, low-pressure form which, as a practical matter, cannot be recovered by further expansion in the turbine. In a simple cycle system this energy is lost to the atmosphere when the gas exhausting from the gas turbine is vented to atmosphere. The magnitude of this energy loss can be appreciated by noting that in a typical simple cycle system, air inducted into the compressor at ambient temperature is heated to a temperature in excess of 1090.degree. C. (2000.degree. F.) in the combustor prior to expansion in the turbine but is only cooled to approximately 540.degree. C. (1000.degree. F.) when vented to atmosphere after expansion in the turbine. Thus, the portion of the fuel burned in the combustor which was used to raise the temperature of the ambient air to 540.degree. C. (1000.degree. F.) is wasted, resulting in poor overall thermodynamic efficiency.
Consequently, substantial effort has been expended in developing methods for recovering the energy available in the gas exhausting from a gas turbine. One of the most successful methods involves the transfer of sensible heat from the hot exhaust gas to pressurized feed water in a heat recovery steam generator (hereinafter HRSG). The HRSG generates steam that is expanded in a steam turbine, thereby producing additional rotating shaft power. Since steam turbines operate on the Rankine cycle, rather than the Brayton cycle, power plants employing such a heat recovery method are termed combined cycle power plants.
Typically, a HRSG is comprised of a large duct through which the exhaust gas flows. The duct encloses banks of tubes through which the water/steam flows and over which the gas turbine exhaust gas flows. The surfaces of the tubes provide heat transfer surfaces. There are three basic components in which heat is transferred in a typical HRSG, each comprised of a bundle of tubes: an economizer in which the feed water is heated to near-saturation temperature; an evaporator in which the water heated in the economizer is converted to steam; and a superheater in which the temperature of the saturated steam from the evaporator is raised into the superheat region.
In order to obtain maximum efficiency of the steam turbine, it is desirable to generate steam at a high temperature and pressure. However, unless supplemental fuel is burned in the exhaust gas, an inefficient practice, the steam temperature is limited to the temperature of the exhaust gas entering the HRSG. The maximum pressure of the steam is also limited by the temperature of the exhaust gas since the saturation temperature of steam increases with its pressure and only the portion of the heat in the exhaust gas which is above the saturation temperature of the water in the evaporator can be used to generate steam. Hence, although increasing steam pressure increases steam turbine efficiency, it also reduces the quantity of the steam generated.
One approach to maximizing heat recovery by steam generation involves the use of a HRSG that generates steam at multiple pressure levels by employing a separate evaporator at each pressure level. The steam generated at each pressure level is then inducted into the appropriate stage of the steam turbine. According to this approach, the gas turbine exhaust gas is directed to the highest pressure evaporator first, then each successive lower pressure level evaporator. Thus, although the temperature of the gas entering the evaporator decreases at each successive pressure level, the saturation pressure (and, hence, saturation temperature of the water) in each successive evaporator is also reduced, so that additional steam may be produced at each pressure level. For example, if steam were generated at three pressure levels, the highest pressure steam is introduced into a high pressure steam turbine, the exhaust from which is combined with steam generated at an intermediate pressure and the combined flow introduced into an intermediate pressure steam turbine. The exhaust from the intermediate steam turbine combines with steam generated at a low pressure and the combined flow introduced into a low pressure steam turbine. The steam is then exhausted from the low pressure steam turbine and condensed and returned to the HRSG. Although this approach results in improved efficiency, the cost and complexity associated with the large intermediate and low pressure steam turbines detracts from its desirability.
Injecting steam into the combustor of a gas turbine has sometimes been used to reduce the NOx generated as a result of the combustion of fuel or to augment the power output of the gas turbine. In the past, such steam injection has been accomplished in a simple cycle gas turbine power plant by generating low pressure steam in a small HRSG and then injecting all of the steam generated into the combustor of the gas turbine. Unfortunately, this approach does not result in the maximum utilization of the heat remaining in the exhaust gas since, at steam low pressure, the exhaust gas is capable of producing more steam than the gas turbine can handle.
In a combined cycle power plant steam injection has been accomplished by generating high pressure steam for the steam turbine and then extracting a portion of the steam mid-way through the turbine and injecting it into the gas turbine combustor. Although this approach increases the recovery of heat from the exhaust gas, such combined cycle plants are complex, involving a large steam turbine, HRSG, condenser, cooling tower, etc., and require a considerable capital investment.
It is therefore desirable to provide an efficient, yet simple and relatively low cost, method of recovering heat from the exhaust gas of a gas turbine by generating steam for injection into the gas turbine.